[FLINK-13793][docs] build each language in a separate subprocess

When not serving docs and just building them, e.g. via `./build_docs.sh`, we
spawn sub-processes for each language build instead of running them
single-threaded (jekyll currently does not support parallel builds).

docs/.gitignore

@@ -3,5 +3,6 @@
 .jekyll-cache/
 .rubydeps/
 content/
+content_*/
 ruby2/.bundle/
 ruby2/.rubydeps/

docs/_config_dev_en.yml

@@ -17,3 +17,6 @@
 
 exclude:
   - "*.zh.md"
+  - "content"
+  - "content_en"
+  - "content_zh"

docs/_config_dev_zh.yml

@@ -17,6 +17,9 @@
 
 exclude:
   - "*.md"
+  - "content"
+  - "content_en"
+  - "content_zh"
 
 include:
   - "*.zh.md"

docs/build_docs.sh

@@ -53,6 +53,8 @@ JEKYLL_CMD="build"
 
 JEKYLL_CONFIG=""
 
+DOC_LANGUAGES="en zh"
+
 # if -p flag is provided, serve site on localhost
 # -i is like -p, but incremental (only rebuilds the modified file)
 # -e builds only english documentation
@@ -78,4 +80,28 @@ while getopts "piez" opt; do
 done
 
 # use 'bundle exec' to insert the local Ruby dependencies
-bundle exec jekyll ${JEKYLL_CMD} ${JEKYLL_CONFIG} --source "${DOCS_SRC}" --destination "${DOCS_DST}"
+
+if [ "${JEKYLL_CMD}" = "build" ] && [ -z "${JEKYLL_CONFIG}" ]; then
+  # run parallel builds for all languages if not serving or creating a single language only
+
+  # run processes and store pids
+  echo "Spawning parallel builds for languages: ${DOC_LANGUAGES}..."
+  pids=""
+  for lang in ${DOC_LANGUAGES}; do
+    bundle exec jekyll ${JEKYLL_CMD} --config _config.yml,_config_dev_${lang}.yml --source "${DOCS_SRC}" --destination "${DOCS_DST}_${lang}" &
+    pid=$!
+    pids="${pids} ${pid}"
+  done
+
+  # wait for all pids (since jekyll returns 0 even in case of failures, we do not parse exit codes)
+  wait ${pids}
+  rm -rf "${DOCS_DST}"
+  mkdir -p "${DOCS_DST}"
+  for lang in ${DOC_LANGUAGES}; do
+    cp -aln "${DOCS_DST}_${lang}/." "${DOCS_DST}"
+    rm -rf "${DOCS_DST}_${lang}"
+  done
+  exit 0
+else
+  bundle exec jekyll ${JEKYLL_CMD} ${JEKYLL_CONFIG} --source "${DOCS_SRC}" --destination "${DOCS_DST}"
+fi